Life History Strategies

The ability of less competitive species to survive when more aggressive species are hindered from reaching maximum productivity by environmental variation or uncertainty has long been realized as fundamental to species diversity. Many attempts have been made to look for combinations of adaptations or strategies that can be used to describe the manner in which different species respond to the pressure of competition for resources or interference from neighbours. The term strategy, when applied to ecology, is divorced from its more common military usage and merely denotes a 'complex of adaptations involving a large number of interacting and coevolving traits' (Calow, 1998). Recently, plant life strategies (for definitions see Table 3.2) have gained new importance in ecological thinking as they can form a basis for conceptualizing the processes that maintain biodiversity (see Chapter 2). However, when these classifications are being applied, as for instance in computer models of vegetation responses to changing environments, it is important to remember that like the concept of competition they are merely human perceptions used to create order from an infinite variety ofplant behavioural responses to the environment and that they are therefore imaginative scenarios rather than biological realities.

3.4.1 Two-class life strategies

The earliest life strategy classifications were based on a two-class division on the use of resources using the terms such as capitalists and proletarians, possibly reflecting radical political thinking at this period (Grime et al., 1988). This and other two-way classifications in essence anticipated the classification of r and K selection strategies where organisms are divided on the basis of whether their lifestyles are adapted to frequently disturbed environments or niches, or whether they are able to exploit continuously highly productive habitats (Fig. 3.23). In r-selected species the typical habitat is one where resources are transient and temporary and thus the population that grows most rapidly prevails. Such populations increase their intrinsic rate of population growth either by increasing their birth rate (seedling production) or more effectively decreasing generation time. Such rapidly growing populations suffer high levels of mortality particularly at the seedling stage. Thus efficient dispersal mechanisms will be favoured, as they will reduce juvenile mortality by distribution of seed to potentially favourable sites. Weed species are sometimes described as being typical r-selected species. However, this is not a universal generalization. Many weed species can be highly competitive and show a degree of ecotype flexibility that allows them to adapt to different situations and operate a successional sequence of their own. This is particularly the case with dangerous perennial weeds such as creeping thistle (Cirsium arvense), field bindweed (Convolvulus arvensis), perennial sowthistle (Sonchus arvensis), couch grass (Elytrigia repens), bracken (Pteridium aquilinum) and field horsetail (Equisetum arvense).

Habitats with long-term stability carry populations of species that are usually close to carrying capacity (K) with little room for further population growth. Such

Table 3.2. Examples of definitions for classifying plants in relation to function

Life-form Classification of species on the basis of morphological features that are associated with their environment. Raunkiaer's scheme (1934), which is based on the height of the perennating bud above the ground during the adverse season, is still the most widely applied system.

Life history traits Features of an organism's life cycle that are most directly related to birth and death rates and thus to Darwinian fitness. Life history traits can be viewed as the way that individuals allocate their limited resources (Calow, 1998).

Life history strategy A grouping of similar or analogous life history characteristics which recur widely amongst species or populations and cause them to exhibit similarities in ecology (Grime, 2001).

Functional types Species or groups of species that have similar responses to a suite of environmental conditions. In practice these groupings are usually related to physiological function as it is affected by different environments.

Fig. 3.23 Two, three and four classification strategies for interpreting plant responses to competition and environmental stress. (a) The basic two classification system between r and K selection. (b) The triangular classification model of Grime between competitors (C), stress tolerators (S) and ruderals (R). (c) The tetrahedral modification ofOksanen and Ranta, 1992, with the three primary strategies (K, r and g) on the face of the tetrahedron. The g strategy is typified by ericoid and graminoid species with low root to shoot ratios and low stature. The additional strategy s lies behind the front plane and represents decreasing allocation to support tissues, with the extreme type being a hepatic. (Reproduced with permission from Oksanen & Virtanen, 1997.)

Fig. 3.23 Two, three and four classification strategies for interpreting plant responses to competition and environmental stress. (a) The basic two classification system between r and K selection. (b) The triangular classification model of Grime between competitors (C), stress tolerators (S) and ruderals (R). (c) The tetrahedral modification ofOksanen and Ranta, 1992, with the three primary strategies (K, r and g) on the face of the tetrahedron. The g strategy is typified by ericoid and graminoid species with low root to shoot ratios and low stature. The additional strategy s lies behind the front plane and represents decreasing allocation to support tissues, with the extreme type being a hepatic. (Reproduced with permission from Oksanen & Virtanen, 1997.)

species can have low levels of fecundity which is offset by a longer life span. Selection for the particular competitive strategies makes K-selected species highly specialized and consequently more sensitive to environmental change. They are also less able to recover from low population densities and are therefore likely to be species in danger of extinction. Vegetative reproduction can enhance the probability of survival by conferring almost virtual immortality. Such cases are seen in many northern populations of aspen which have had difficulty in setting seed during the Little Ice Age (a period of cooling lasting approximately between the thirteenth and nineteenth centuries) but nevertheless have survived due to an ability to sucker. Similarly, stands of trees lying outside the main boundaries for forest regeneration are maintained by layering. Some alien species achieve pest status due to their capacity for vegetative reproduction. In Scotland the common rhododendron (Rhododendron ponticum) was introduced to the Highlands and owing to its capacity for layering and shade tolerance is able to outcompete much of the native woodland flora.

3.4.2 Three-class life strategies

Although simple binary divisions are sufficient to highlight some of the most contrasting dichotomies in competition behaviour in both plants and animals there is considerable improvement when plant competition is viewed through three-strategy models.

A variety of additional third classes of plant behaviour have been suggested. Adversity selection (Southwood, 1988) brings into play the quality of the habitat where stressful conditions can be accommodated at a metabolic cost to the organism but results in impaired growth or reproduction. The notion of a third dimension in competition strategy as developed by Grime recognizes differences in mortality between the behaviour exhibited by juvenile as opposed to adult and established plants. Variations in regenerative strategies exploit differences in resource investment, mobility and dormancy which are all expressed mainly in the regenerative phase. This has led to the suggestion (Grime et al., 1988) that there are two major classes of external factors which affect the life strategies of plants. The first may be described as stress and consists of all those phenomena which can restrict photosynthetic production, such as shortages of light, water and mineral nutrients or suboptimal temperatures.

The second class is referred to as disturbance and covers all phenomena which result in the partial or total destruction of biomass such as herbivory, human disturbance through trampling, mowing and natural physical disturbances from wind damage, frost drought, soil erosion and fire. In the three-class life strategy concept, it is asserted that plants can occupy only two extreme marginal conditions, namely either extreme stress or extreme disturbance. The combination of both extreme stress and extreme disturbance is considered by Grime to result in areas that are not habitable by plants.

3.4.3 Four-class life strategies

There are, however, situations in some marginal habitats that have prompted the suggestion that it is possibly more realistic to visualize plants as having up to four basic life strategies. In some arctic, subarctic and montane habitats it is possible to have intense grazing in a severely resource-limited environment and survive (see Section 10.3). In many instances it can even be demonstrated that this grazing is essential for the survival of flowering plants in order to avoid buildup of litter and shading and the consequent formation of lethal ice wedges in the soil.

Even in relatively unproductive arctic and alpine habitats, grazing has always existed and is frequently accompanied by intense physical disturbance through cryoperturbation and erosion. It has been argued that grazing selects for entirely different kinds of traits from devastating physical disturbance, which leaves no survivors in situ (Oksanen, 1990, 1993). Loss of above-ground tissues obviously selects for high root to shoot ratios and for vegetative reproduction by means of horizontal rhizomes, especially if the loss is caused by grazers, which prefer nutrient-rich floral shoots. Moreover, herbivory selects for tough, narrow, finely lobed or scale-like leaves and against broad, mesomorphic leaves. Grazing is particularly detrimental to the latter as partial consumption of broad mesomorphic leaves creates long wounds, exposing the remaining tissues to desiccation and invasion by parasitic fungi.

The concept that plants have evolved certain stable defence strategies suggests that there are two main strategies in relation to withstanding grazing. Plants can either invest large amounts of reduced carbon in defensive compounds, which inevitably results in low growth rates, or else they can defend themselves solely by means of readily available inorganic substances (e.g. silica) or by means of mechanical deterrents (e.g. thick cuticles, thorns), which are relatively efficient even at low levels of resource investment. The two main ways of surviving intense grazing have been described as ericoid and graminoid strategies (Oksanen, 1990). The main advantage of typical graminoids - their ability to produce erect leaves rapidly from basal intercalary meristems - is claimed by Oksanen to be reduced in habitats with persistent, intense grazing, where shoot competition never becomes intense. The above four-class strategies have properties that can be compared with each other by an expansion of Grime's triangular strategy diagram into a tetrahedron (Oksanen & Ranta, 1992). One corner of the scheme represents the K-strategy or competitiveness sensu Tilman (Tilman, 1988), i.e. a suite of morphological, physiological and reproductive traits which enables plants to complete their life cycles in an environment where critical resources are depressed to a low level by these plants. Another pole is the classical r-strategy, which can be regarded as identical to Grime's R or ruderal strategy. Grime's competitors or C-strategists, which are adapted to resource-rich environments where interference competition prevails, are regarded as intermediate between K- and r-strategists, as proposed by Grace (Grace, 1990). The third pole is formed by g-strategists, adapted to frequent but small losses of above-ground tissues. The s-strategy represents decreasing allocation to support tissues with the extreme type being a hepatic. This system also envisages further subdivisions with ericoid, graminoid, and Dryas strategies, being subsets of the more general concept of g-strategy (Oksanen & Virtanen, 1997).

Despite its conceptual complexities, this discussion of a four-class classification of plant life strategies as compared with the more traditional concept of two-and three-class strategies serves to demonstrate that as conditions become more marginal for plant survival this need not be accompanied by any loss in diversity of either form or function in higher plants. As is argued throughout this book, plants surviving in marginal areas are not necessarily impoverished in the diversity of their adaptations.

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